Effects of Zb states and bottom meson loops on γ(4S)→γ(1S, 2S)π+π− transitions

摘要: We study the dipion transitions (4S) → (nS)π+π− (n = 1, 2). In particular, we consider the effects of the two intermediate bottomoniumlike exotic states Zb(10610) and Zb(10650) as well as bottom meson loops. The strong pion–pion final-state interactions, especially including channel coupling to K ¯K in the S-wave, are taken into account modelindependently by using dispersion theory. Based on a nonrelativistic effective field theory we find that the contribution from the bottom meson loops is comparable to those from the chiral contact terms and the Zb-exchange terms. For the (4S) → (2S)π+π− decay, the result shows that including the effects of the Zb-exchange and the bottom meson loops can naturally reproduce the two-hump behavior of the ππ mass spectra. Future angular distribution data are decisive for the identification of different production mechanisms. For the (4S) → (1S)π+π− decay, we show that there is a narrow dip around 1GeV in the ππ invariant mass distribution, caused by the final-state interactions. The distribution is clearly different from that in similar transitions from lower states, and needs to be verified by future data with high statistics. Also we predict the decay width and the dikaon mass distribution of the (4S) → (1S)K+K− process.